Fluid containment structures such as storage tanks and pipes are used in numerous settings to store and channel fluids such as water and fuel. It is highly desirable, and in some situations imperative, that such containment structures be periodically cleaned to remove impurities that have accumulated on interior surfaces during use. A thorough cleaning of a fluid containment structure often requires access to the interior of the structure so that fluid may be removed from the structure and the interior surfaces of the tank walls cleaned using pressurized fluid.
While the present invention has broad use to any fluid containment structure in which impurities may accumulate, it is of particular importance when used to allow access to fluid storage tanks such as water tanks and fuel tanks. Accordingly, the present invention will be explained in the context of a fluid storage tank, with the understanding that the scope of the present invention is limited by the claims appended hereto and not the following detailed description.
Often, fluid storage tanks are constructed and installed without an opening suitable for allowing the interior surfaces of the tank walls to be cleaned. In such cases, an opening must be created in the tank to allow cleaning, and a plate must be installed onto the tank to prevent loss of fluid when the tank is not being cleaned.
Further, fluid storage tanks are often installed in constricted spaces. For example, in the shipbuilding art, vessels are often built around storage tanks for water and fuel. In such situations, only a portion of the tank may be exposed, and this exposed portion of the tank may immediately adjacent to other immovable objects that render it difficult to work on the exposed portion of the tank.
A number of methods are known for providing access to fluid storage tanks. In one such method, which will be referred to herein as the sheet metal screw method, an access hole is formed in the tank wall and small mounting holes are formed around the access hole. The access plate that covers the access hole is attached to the tank wall using sheet metal screws that engage the mounting holes surrounding the access hole. A gasket is provided to seal the gap between the access plate and the tank wall.
However, the tank walls are often relatively thin and soft, and the screws used to attach the access plate to the tank wall tend to enlarge the mounting holes over time. At some point, the mounting holes no longer effectively engage the screws to hold the access plate on the tank wall. The sheet metal method will eventually fail and is inappropriate for use in many environments.
Another assembly for providing access to the interior of fluid storage tanks is shown at 20 in FIG. 1 and will be referred to herein as the welding method. This assembly comprising a rigid ring 22 welded onto an external surface 24 of a storage tank wall 26 around an access opening 28. An access plate 30 is mounted onto the tank wall 26 over the access opening 28 by an attachment assembly comprising studs 32, flat washers 34, lock washers 36, and nuts 38. The studs are permanently mounted on the rigid ring 22. A ring gasket 40 is arranged between the ring 22 and the access plate 30.
The assembly 20 shown in FIG. 1 is highly reliable. However, the use of welding to attach the rigid ring 22 onto the tank wall 26 significantly increases the costs of providing access to the interior of a storage tank; the owner of the storage tank may not have the equipment or skill to weld the rigid ring 22 onto the tank wall 26.
Further, if the tank formed by the wall 26 is used to store flammable fluids, these fluids must be removed and the tank filled with inert gas prior to welding. This greatly increases the cost and complexity of allowing access to the interior of the tank.
Also, as mentioned above, the wall 26 of the storage tank may be in a place that is inaccessible to welding machinery. If this is the case, the welding method may not be available or only at the great cost of removing either the tank or the structure blocking access thereto.